1. Field of the Invention
The present invention is directed to a method and system for fabricating low defect-density oxides, and more particularly to a method and system for fabricating oxides using at least one of a pulsed RF energy source and a pulsed process gas source.
2. Discussion of the Background
Significant attention has been given to methods of forming the gate dielectric layer used in MOSFETs. Both oxidation of semiconductors and deposition of dielectric layers on the surface have heretofore been used. Thin silicon dioxide films have been formed by various methods such as thermal oxidation, low pressure chemical vapor deposition, photo-activated chemical vapor deposition, plasma enhanced chemical vapor deposition, and plasma oxidation. The most successful approach for gate oxide growth so far has been thermal oxidation of silicon at elevated temperatures (e.g., in excess of 900° C.). High temperature thermal oxidation of silicon provides a thin layer of low defect density silicon dioxide as well as a good silicon/oxide interface.
One critical property of the gate oxide layer, regardless of the way it is fabricated, is that it must have high dielectric strength, or in another words, very low defect density. Dielectric strength is directly, but inversely, related to the defect density inside the dielectric layer. Defects in the gate oxide can be formed because of numerous reasons (e.g., particulates, defects that originally resided at the silicon surface, surface contamination, and atoms arriving at the reaction front not having enough energy to move into full density location sites). Assuming the process starts with a perfect silicon surface and free of particulates and contaminants, the defect density of the resulting silicon dioxide is very much dependent upon the energies of the atoms arriving at the reaction surface. This is true for both oxidation and deposition processes.
As the ULSI technologies advance, the requirements for precise dopant distribution become extremely stringent. Processes requiring high temperature operation (e.g., at least 900° C.) for a substantial length of time (e.g., at least 5 minutes) may cause dopant re-distribution and, therefore, they should be minimized wherever possible. As a result, approaches that perform oxidation or annealing at temperatures lower than 800° C. or for a period of time shorter than a few minutes become very attractive.
Growing silicon dioxide with the assistance of a plasma discharge provides a method of fabricating silicon dioxide at temperatures as low as 300° C. Plasma assisted growth of oxides can be further classified as either: (1) plasma oxidation or (2) plasma enhanced chemical vapor deposition.
U.S. Pat. No. 4,232,057 (hereinafter “the '057 patent”) describes a method of growing oxide films on silicon utilizing plasma oxidation. The process employs oxygen at a flow rate of 0.7–10 sccm utilizing a relatively high RF power. The substrate is held at a temperature between 300°–800° C. and about 20 cm away from the plasma source, as shown in FIG. 1. The oxidation rate was found to be dependent upon the RF power applied to the plasma source, as shown in FIG. 2. According to the method, the main oxidation species is oxygen radicals generated by the plasma discharge. The oxygen radicals diffuse into the silicon substrate and react with silicon to form an oxide on the surface. Although the oxygen radicals are chemically active, they carry very little energy. The mobility of the atoms on the silicon surface is therefore mainly controlled by the substrate temperature. As a result, the formed silicon oxide is more porous and defective. It is also known that the use of high RF power results in thicker oxide films with poor quality.
U.S. Pat. No. 5,412,246 describes a method for growing thin oxides as gate dielectric layers using plasma discharge and an ozone/oxygen gas mixture. The ozone is generated in an ozone generator separated from the RF plasma reactor. Using the stand-alone ozone generator eliminates the need for high RF power in the oxidation chamber that may result in poor oxide quality. The use of RF power is optional as ozone itself can provide chemically active oxygen species for oxidation. Low RF power is used to enhance the oxide growth rate and to improve film quality and thickness uniformity. The low power RF (<200 watt/cm) can reexcite or redistribute the ozone. A supplemental rapid thermal oxidation at temperatures of 700°–800° C. in the presence of ozone may be introduced after the plasma oxidation as a follow-up. The dielectric strength of the resulting oxide, as shown in FIG. 3, shows high film quality that is comparable to that obtained by high temperature thermal oxidation. Similar to the work described in the '057 patent, the oxidation of the silicon surface relies on the chemically active oxygen species and the mobility of the atoms on the surface which is only provided by the thermal heating of the substrate. The resulting oxide film may be more porous and may depend heavily on the initial surface condition.
U.S. Pat. No. 4,776,925 describes producing silicon oxide or nitride thin films using low energy ion beam bombardment. As shown in FIG. 4, the method directs an ion beam having an energy level around 60 eV to a silicon substrate at room temperature. The ion beam is formed by an ionized gas of oxygen or nitrogen, possibly diluted with argon, in a glow discharge. The ions are then extracted through a grid biased negatively with respect to the plasma. A similar approach to forming silicon oxide layers using oxygen ion beams is disclosed in U.S. Pat. No. 5,122,483. Disadvantageously, those energetic ion beams may result in high surface contamination because of the ion sputtering effect. The re-deposition of sputtered materials onto the substrate surface may also degrade the oxide quality. Furthermore, the throughput of oxidizing large diameter silicon wafers is very low using a single ion beam technique.
A similar concept of using atomic oxygen to react with a silicon surface at elevated temperatures without plasma discharge has also been studied. U.S. Pat. Nos. 4,474,829, 4,409,260, and 5,738,909 generally use either ozone or UV radiation excited oxygen or oxygen-containing precursors to create the atomic oxygen. A high substrate temperature or an RTP process is needed in order to obtain gate-quality oxide.
Plasma enhanced chemical vapor deposition has been used to fabricate relatively thick silicon dioxide or silicon nitride layers (e.g., often >1000 A), primarily because of its high deposition rate (>3000 A/minute) and low process temperature (<500° C.). The atoms or molecules arriving at the surface in a conventional PECVD system do not have enough energy and time to diffuse into a low energy position. Therefore, the dielectric films generated generally have a high defect density and a low electric breakdown field.
Plasma oxidation and plasma enhanced chemical vapor deposition have been used to produce silicon oxide as dielectric layers. As described above, these plasma enhanced processes have the advantage of fabricating silicon oxide at low temperatures (e.g., below 800° C.). However, gate-quality films of less than 100 A produced by plasma oxidation or PECVD have not been achieved without utilizing other supplemental processes such as rapid thermal oxidation. Although oxygen ion beams have been used to produce thin oxide films as gate dielectrics, the use of relatively high-energy ion beams has the disadvantages of low throughput, possible surface damage and sputtering contamination.